Physicochemical and morphological characterization of a glass ceramic treated with different ceramic primers and post-silanization protocols

Highlights

• Primers produced different physical and morphological surface characteristics.

• Post-silanization protocols’ effects on surface properties is primer dependent.

• Wash-and-dry and hot air protocols created a promising surface to chemically bond.

• Ceramic primers effectively formed chemical bonds with ceramic surface.

Abstract

Objective

Evaluate the effect of different ceramic primers and post-silanization protocols on physicochemical and morphological characteristics of a lithium disilicate glass ceramic.

Methods

Lithium disilicate ceramic (IPS e-max CAD) plaques (6 × 10 × 2 mm) were divided into 3 groups according to the ceramic primer used: (1) Silane (RelyX Ceramic Primer-RL); (2) Silane + MDP (Clearfil Ceramic Primer Plus-CP); (3) Self-etching ceramic primer (Monobond Etch and Prime-MB). Specimens from each group were distributed into 5 sub-groups according to post-silanization protocols: (a) Treated as recommended by the manufacturer (MR), (b) MR + Additional drying with air at room temperature for 30 s (RTA), (c) MR + additional drying with hot air for 30 s (HT), (d) MR + Surface rinsing with water at room temperature for 10 s and drying with air at room temperature for 30 s (WT), and (e) Specimens were not silanized (NS). Surface free energy (SFE) was determined using static contact angles measurements with water and diiodomethane. SFE data were submitted to Friedman followed by Wilcoxon post-hoc test (α = 0.05). Morphology was analyzed using scanning electron microscopy. Elemental composition and chemical interactions were determined with X-ray photoelectron spectroscopy analysis.

Results

RL presented the highest SFE (62.4 mN/m) followed by CP (59.7 mN/m). Post-silanization protocols resulted in similar SFE, but WT and HT induced the highest water contact angles when using CP and RL. CP modified ceramics’ surface morphology compared to the etched and RL treated groups. The presence of water was identified on CP treated specimen. All analyzed primers formed siloxane bonds with ceramic surface.

Significance

Ceramic primers resulted in different surface free energy and morphology, but siloxane bonds were identified for all tested solutions. HT and WT protocols should be used with RL and CP primers. MB was not influenced by the different silanization protocols.

Introduction

A wide range of ceramics with different compositions and processing techniques are now available for indirect restorations [1,2]. Different classification systems have been proposed. For example, dental ceramics can be classified according to the glass content on its chemical composition as polycrystalline or glass ceramics [3]. Lithium disilicate glass ceramics are mainly composed of a silica rich phase reinforced with crystals and have been widely used due to optimal mechanical properties and aesthetics [2,4]. Moreover, they can be adhesively luted to the tooth structure. A strong and long-lasting bond with resin cement depends on the performance of a proper ceramic surface treatment [2,3,5]. Two main approaches have been recommended to prepare the ceramic restoration: micromechanical retention and chemical bond [3]. Hydrofluoric acid changes surface topography as it selectively attacks the glassy phase increasing surface energy and creating microporosities which facilitate micromechanical interlocking of the resin cement [2,3]. On the other hand, silane coupling agents act as mediators and promote chemical bonding of the resin cement with the ceramic surface [1].

Silanes consist on organic compounds that essentially contain one or more silicon atoms [6]. The most common type used in dentistry is ɣ-methacryloxypropyltrimethoxysilane (MPTS). It is a bifunctional molecule with one organofunctional group containing methyl methacrylate that copolymerize with resin cements and hydrolysable alcoxy groups that reacts with Si−OH on the ceramic surface [1,7]. The alcoxy groups must first be activated via hydrolysis (SiOR→SiOH) in order to be suitable to chemically react with ceramic material [8,9]. The silanol group suffers a condensation reaction with hydroxyls present on the ceramic, releasing water as a byproduct [10]. MPTS also reacts within to form siloxane bonds by horizontal condensation, resulting in a cross-linking tridimensional layer [11]. The formation of this branched hydrophobic layer increases hydrolytic stability and makes the ceramic surface more prone to bond resin materials [5,12].

The structure of the silane layer is influenced by a number of factors such as composition of the silane solution and post-silanization protocols [13]. These protocols should remove water, solvents and contaminants in order to increase the accessible sites on ceramic surface to bond to MPTS molecules and enhance the condensation reactions within the silane compounds [8]. The efficiency of different protocols might vary according to each silane composition [14]. Manufacturers usually recommend the application of a mild oil-free air, but it seems that is not enough to effectively remove solvents [13,15]. The usage of heat treatment has been the main alternative treatment previously evaluated, but the effectiveness of this protocol is not yet a consensus [13,[16], [17], [18]].

In general, the composition of conventional silanes consists on MPTS molecules, acetic acid, ethanol and water. They are available in one bottle, which is presented in a pre-hydrolyzed solution or in a two-bottle system that should be mixed before use [10]. However, in order to obtain universal and simplified silane primers, new formulations are emerging. Multi-purpose ceramic/metal primers containing MPTS with 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) have been recently released [[19], [20], [21]]. The incorporation of the 10-MDP in these primers is justified by the reduced ability of silanes to form chemical bonds to zirconia and metal reason why phosphate monomers are employed along, as they have been reported to bond successfully to such substrates [22]. Therefore, they could be used with both glass ceramics and zirconia. However, some studies have demonstrated that the association of 10-MDP with silane in a single bottle resulted on deactivation of the silanol component present on MPTS molecules and consequently reducing its bonding potential [20,21]. Moreover, the effect of these primers on glass-ceramics’ properties have not been clarified yet.

A new self-etching ceramic primer was also introduced claiming to etch and silanize in one step [23]. According to the manufacturer, it is composed of tetrabutylammonium dihydrogen fluoride that produces a less aggressive etching pattern compared to HF and along with a silane system based on trimethoxypropyl methacrylate they both form a strong bond with the ceramic surface [23,24]. The effectiveness of this product on ceramic-resin cement bonding is still contradictory [[24], [25], [26], [27], [28]] and detailed characterizations are scarce. Besides the inherent advantage as a simplified treatment, some aspects regarding its effect on ceramic surface and its capability to form durable siloxane bonds, require more investigations.

Thus, the aim of the present study was to evaluate the effect of different ceramic primers and post-silanization protocols on surface energy and morphology of a lithium disilicate glass ceramic and on the chemical composition and interaction of the silane layer formed on ceramics’ surface. The null hypotheses tested in this study are that: (1) there are no significant differences on surface free energy of ceramics treated with different ceramic primers and post-silanization protocols; (2) there are no significant differences on surface morphology of ceramics treated with different ceramic primers and post-silanization protocols; (3) the resultant composition and chemical interaction of the silane layer is not different among ceramic primers and post-silanization protocols.